KR100407116B1 - 3-Layers Coating Method and Apparatus for Steel Pipe and Fittings by Powder Fusion - Google Patents

Abstract

The present invention relates to a coating method and apparatus in which a three-layer coating is sequentially performed by powder welding on the surface of a steel pipe including pipe fittings used for underground laying for transporting liquid, liquefied gas, water, and the like. By holding and rotating both ends of the steel pipe, the jig device that can be used universally regardless of straight pipe or deformed pipe is adopted to make coating work more smoothly and effectively.

The present invention is a pretreatment step of removing foreign matter attached to the steel pipe, a heating step of heating the steel pipe passed through this pretreatment step, a coating step of applying a coating powder to the surface of the steel pipe subjected to this heating step to form a coating layer, this coating In the coating method of the steel pipe including a cooling process for cooling the steel pipe after the process, the coating process is a first coating process for forming a powder epoxy layer on the surface of the steel pipe by powder coating by spraying the steel pipe while rotating, the first coating process By dropping the adhesive modified polyethylene powder from the top of the heated steel pipe in the state of rotating the steel pipe coated with the powder epoxy layer to form a modified polyethylene adhesive layer while the modified polyethylene powder dropped on the surface of the powder epoxy layer of the steel pipe is welded When rotating the steel pipe coated with the modified polyethylene adhesive layer in the second coating process and the second coating process Is characterized in that the third coating step of forming a polyethylene resin layer as a top of the polyethylene resin powder falling on a modified polyethylene adhesive layer to the polyethylene resin powder falling from the upper part of the steel pipe welded in a state formed by one.

Description

3-Layers Coating Method and Apparatus for Steel Pipe and Fittings by Powder Fusion}

The present invention relates to a method and apparatus for surface coating of steel pipes including pipe fittings in the form of a deformed pipe that is used for underground laying for transporting liquid, liquefied gas, water, and the like, and more specifically, powder coating of steel pipe surface coatings. The present invention relates to a three-layer coating method and apparatus for steel pipes in which a three-layer coating is performed sequentially.

In general, in order to protect the surface of the steel pipe is a protective coating on the surface of the steel pipe and the protective coating method is proposed in the Korean Industrial Standards KSD3607 and KSD3589.

In the KSD3607, the KSD3607 is subjected to a pretreatment process of mechanically or chemically removing oil, rust, and other foreign matters attached to the tube, and then heats the tube to a constant temperature and then melts the polyethylene in powder form. The cover method is adopted.

In the case of KSD3589, after the pretreatment, the outer surface of the tube is heated in advance to apply an adhesive and to cover the polyethylene layer by extrusion.

That is, the conventional coating method is divided into powder welding and extrusion type, and most of them are protective coating of one or two layers on the outer surface of the tube.

However, in such a conventional coating method, the surface protective coating is composed of only polyethylene coating layer, so that corrosion resistance is poor and the service life of steel pipes is relatively shortened, causing problems in use.

In order to solve this problem, the coated steel pipe has been recently spotlighted. The protective coating on the surface is made of three layers of a powder epoxy epoxy layer, a modified polyethylene adhesive (PE adhesive) layer and a polyethylene (PE) resin layer. Coated steel pipes have been developed and utilized. The existing coating method for forming such a three-layer coating is mainly adopted to the coating method by the extrusion method.

However, since the conventional three-layer coating method is made by the extrusion method, the original pipe can be applied to the steel pipe of the straight pipe shape, but in the case of the release pipe of irregular shape, the extrusion die cannot be used for the entire steel pipe. In addition, even in the case of straight pipes, the extrusion coating method requires a large amount of facility cost to prepare the main equipment and accessories for coating work, especially the diameter of the steel pipe. In the case of the large diameter of more than 1500mm, the facility investment due to the enlargement of the size is exponentially increased, so it is difficult to actually adopt.

The present invention has been researched and developed in order to solve the above-mentioned conventional problems, and the object thereof is listed in detail.

It is an object of the present invention to provide a three-layer coating method and apparatus that can be applied to the form of a straight pipe as well as a straight pipe, as well as a deformed pipe corresponding to a pipe fitting, and can be universally applied to various types of steel pipes in small quantities. .

Another object of the present invention is to form a coating layer consisting of the first layer to the third layer sequentially and to achieve a solid and reliable welding while greatly reducing the equipment cost of the facility required to perform the coating operation The present invention provides a three-layer coating method and apparatus.

Another object of the present invention is to adopt a jig device that can be used universally regardless of straight pipes or deformed pipes while rotating while holding both ends of the steel pipe during the coating work to make the coating work more smoothly and effectively To provide a layer coating method and apparatus.

1 is a block diagram showing to explain a preferred example of the coating method according to the invention,

Figure 2 is a schematic plan view showing a three-layer coating method and apparatus by the powder fusion of the present invention,

Figure 3 is a plan view showing an extract of the heater portion in the apparatus of the present invention,

Figure 4 is a front view showing a part cut in Figure 3,

Figure 5 is a plan view showing an extract of the bogie and jig device commonly used in the coating process in the apparatus of the present invention,

6 is a front view of FIG. 5;

7 is an enlarged view showing in detail an extract of only one side of FIG.

8 is a plan view of the primary coating device showing a state equipped with a steel pipe of the form of a release tube in the device of the present invention,

9 is a front view of FIG. 8;

10 is a plan view of the second, third coating device showing a state in which the steel pipe of the straight pipe and the release pipe in the jig device of the trolley in the device of the present invention;

FIG. 11 is a front view of FIG. 10; FIG.

{Description of symbols for main parts of the drawing}

1: steel pipe 2: pretreatment device 3: heating device 4. Coating device 5: Jig device 6. First coating device

7. Second coating device 8. Third coating device 9: Cooling device 10: Rail 20: Truck 30: Heating furnace 32: Burner 50: Fixed frame 52: Rotating drive body 53: Rotating reciprocating member 55: Angle adjusting member 56: Support plate 57: Jig 58: Fixing member 60, 70: Coating chamber 62: Nozzle 76: Powder application mechanism 79: Collection mechanism 80: Feeding mechanism 82,86: Screw conveyor 87: Bucket conveyor

The present invention for achieving the above object is a pretreatment step of removing abrasives and dust attached to the steel pipe, a heating step of heating the steel pipe after this pretreatment step, powder for coating on the surface of the steel pipe subjected to this heating step In the coating method of a steel pipe comprising a coating step of coating a coating layer to form a coating layer, and a cooling step of cooling the steel pipe subjected to the coating step, the coating step is powder coating by spraying the powder while spraying the steel pipe on the surface of the steel pipe. The modified polyethylene powder is deposited on the surface of the powder epoxy layer of the steel pipe by dropping the modified polyethylene powder from the top of the heated steel pipe while rotating the steel pipe coated with the powder epoxy layer by the first coating process and the first coating process to form a layer. In the second coating step of forming a modified polyethylene adhesive layer while the powder is welded and in the second coating step A third coating process is performed in order to form a polyethylene resin layer while the polyethylene resin powder which contacts the surface of the modified polyethylene adhesive layer is welded by dropping the polyethylene resin powder from the upper portion of the steel pipe in a state of rotating the steel pipe having the polyethylene glycol adhesive layer formed thereon. It is characterized by.

Further, a pretreatment apparatus for removing abrasives or dust attached to the steel pipe, a heating apparatus for heating the steel pipe pretreated by the pretreatment apparatus, a coating apparatus for forming a coating layer on the surface of the steel pipe heated by the heating apparatus, this coating A coating device comprising a cooling device for cooling a steel pipe coated by a device, wherein the coating device comprises a pair of jig devices that are rotated and mounted on an upper portion of a trolley while the steel pipe is mounted thereon; A first coating device for forming a powder epoxy layer as a first coating layer, a second coating device for forming a modified polyethylene adhesive layer as a second coating layer, and a third coating device for forming a polyethylene resin layer as a third coating layer. It features.

Hereinafter, the present invention will be described in more detail based on the accompanying drawings of preferred embodiments.

First, referring to FIGS. 1 and 2, the present invention is a three-layer coating method according to a sequential process.

First, the pretreatment process is performed as in the conventional steel pipe coating method, and this pretreatment process cleanly removes abrasives or dusts on the surface of the steel pipe by a conventional shot blasting method or other appropriate method. After the pretreatment process, the steel pipe is heated in a heating furnace, and the heating temperature at this time is a temperature at which the coating powder, which is a material to be coated, is melted on the surface of the steel pipe, and thus the close coating can be performed. The coating powder is heated higher than the melting temperature of the coating powder. After the heating process as described above, the coating powder is coated on the surface of the steel pipe to form a coating layer, and the coated steel pipe is immersed in the cooling water tank or the steel pipe is coated on the surface of the steel pipe. The steel pipe is forcibly cooled by, for example, spraying cooling water. The reason for forcibly cooling the steel pipe is to form a film with a uniform thickness and a uniform film structure on the surface of the steel pipe.

The coating step includes a first coating step of forming a powder epoxy layer as a first coating layer on an outer surface of the steel pipe, a second coating step of forming a modified polyethylene adhesive layer as a second coating layer on the first coating layer, and the second coating layer. A third coating step of sequentially forming a polyethylene resin layer as a third coating layer is performed.

In detail with respect to the coating process, the first coating process is performed by powder coating by spraying while the steel pipe is rotated,

In the second coating process, the modified polyethylene powder having good adhesion is naturally dropped from the upper portion of the steel pipe heated to a predetermined temperature while continuously rotating the steel pipe coated with the powder epoxy layer in the first coating process. While the powder is in contact with the powder epoxy layer on the surface of the steel pipe is welded by the heat of the steel pipe and powder epoxy layer to form a modified polyethylene adhesive layer,

In the third coating process, the polyethylene resin powder is naturally dropped from the upper portion of the steel pipe in the state of rotating the modified polyethylene adhesive layer coated steel pipe in the second coating process, as in the second process. While contacting the modified polyethylene adhesive layer of the polyethylene resin powder is deposited by the heat of the steel pipe and the coating layer on its surface to form a polyethylene resin layer.

Next will be described with respect to the coating apparatus capable of sequentially performing the three-layer coating method according to the present invention described above.

2 is a schematic diagram of a three-layer coating apparatus according to the present invention, in which a heating device 3 is installed in front of the planar pretreatment device 2, and the first coating device 6 and the second coating device from the heating device 3. The coating device 4 including the third coating devices 7 and 8 is arranged in a straight line, and a cooling device 9 is installed at the rear of the third coating device 8.

Since the pretreatment device 2 is a conventional sand blasting machine for strongly injecting sand into high pressure air or the like, a detailed description thereof will be omitted.

3 and 4, the heating device 3, the opening and closing door 34 is installed at the inlet and outlet of the heating furnace 30 and a plurality of burners 32 are installed on both sides of the heating furnace 30, In the bottom of the heating furnace 30, a rail 10 is provided at the inlet of the coating device 4 and passes through the third coating device 8, and the rail 10 has a drive mechanism 20 having a drive mechanism. Is installed. The heating device 3 having the above-described configuration drives the trolley 20 in a state where the steel pipe 1 is mounted on the trolley 20 to enter the inside of the heating furnace 30 and the steel pipe by the burner 32 ( 1) is heated to a predetermined temperature and then moved to the outside of the heating furnace (30).

The drive mechanism of the trolley 20 is provided with a plurality of rotatable wheels 24 rotatable on the frame 22 to move the trolley 20 on the rails 10, and some of these wheels 24 are trolley 20. The drive motor 26 is installed to enable self-drive.

The trolley 20 used for the coating device 4 is provided with a pair of jig devices 5 for mounting the steel pipe 1 on both sides, and the jig device 5 has a brake 50a. A fixed frame 50 installed on the base 50b and having a housing 51 at the upper end thereof, and a drive motor 52a inside the housing 51 of the fixed frame 50, is rotatable and provided on the inner surface. A spline portion 53b is formed on the outer surface of the rotary drive body 52 with the spline portion 52b and the spline portion 52b of the rotary drive 52, and the hydraulic cylinder 53a at the rear side. Rotational reciprocating member 53 is connected to enable the forward and backward movement by. The end of the piston rod 53c and the rotary reciprocating member 53 of the hydraulic cylinder 53a are connected through a bearing 53d so that the rotary reciprocating member 53 is not affected even when the rotary reciprocating member 53 is rotated. 50, the lower member 50d is fixed to the base 50b to the upper part, and the upper member 50c having the housing 51 on the upper end of the lower member 50d is liftably fitted, and the upper member 50c is provided. The hydraulic cylinder 50e for height adjustment is connected between the base 50b and the base 50b so that the rotary drive member 52, the rotary reciprocating member 53, and the like can be lifted. The support plate 56 is connected through the branch portion 54, and the support plate 56 is connected to the lower side of the rotary reciprocating member 53 by an angle adjusting member 55 so that the angle can be adjusted. 55 is a turnbuckle in which the screws with different screw directions are fastened to both sides of the adjuster 59, According to the rotation direction of the adjuster 59, the screw is extended or contracted in the longitudinal direction thereof to adjust the angle between the rotating reciprocating member 53 and the support plate 56 about the hinge portion 54. The support plate 56 The outer side of the) is provided with a fixing member 58 to enable the slide movement, the fixing member 58 is provided with a plurality of jigs 57 to expand or contract radially from the center thereof.

The first coating device 6 of the coating device 4 is provided with a plurality of nozzles 62 for spraying the coating powder on both sides of the coating chamber 60.

The second and third coating apparatuses 7 and 8 of the coating apparatus 4 are the same as each other and are denoted by the same member numbers for the same constituent members for convenience, and have a plurality of columns inside the coating chamber 70. 71) A fixed frame in which a crosspiece 72 crosses an upper portion and an arm 73 extends on the crosspiece 72, and a powder coating installed to adjust the height while hanging on the lower portion of the arm 73 of the fixed frame. A collecting mechanism 79 having a mechanism 76, an inclined surface 77 at the lower portion of the coating chamber 70, and a powder collecting space 78; and a powder collected by the collecting mechanism 79 A feeding mechanism 80 for feeding the application mechanism 76 is provided.

The powder applying mechanism 76 is connected to the winding rod 85b rotated by the forward and reverse motor 85a of the crossbar 72 and the end of the wire 85c wound on the winding rod 85b, and the winding rod is installed. Height can be adjusted by winding and unwinding the wires 85c and 85c according to the direction in which the 85b is rotated. In addition, the powder coating mechanism 76 is a bidirectional screw conveyor 82 is installed in the inner space (76a) to spread the powder evenly in the longitudinal direction, the screw conveyor 82 is driven by a drive motor 81 The opening and closing slots 83 are formed in the lower portion of the entire screw conveyor 82 in the longitudinal direction.

The collecting device 79 is a bidirectional screw conveyor (79a) is installed so that the powder collected in the powder collecting space 78 is collected in the center, and collect the powder in the lower center of the powder collecting space (78) A partition space 79b for feeding is provided.

The feeding mechanism 80 is configured to feed the powder from the compartment space 79b to the powder application mechanism 76, and is arranged at the rear end of the screw conveyor 86 installed in the compartment space 79b. 71, the bucket conveyor (87) is installed in a standing state at the rear of the bucket conveyor (87), the screw conveyor (88) installed horizontally to the upper end of the powder coating mechanism (76) is connected and installed. A corrugated pipe 89 is connected to the end of the screw conveyor 88 and the upper center portion of the powder applying mechanism 76 to communicate with each other.

Next, based on the three-layer coating apparatus of the present invention described above, the present invention will be described in detail the three-layer coating method.

That is, the three-layer coating method of the present invention will be described in the order of pretreatment step, heating step, first to third coating step, and cooling step.

Pretreatment process

In the case of the pretreatment process, the steel pipe (1) prepared in advance for coating is placed inside the pretreatment device (2), regardless of the shape of the deformed pipe or the straight pipe, and the steel pipe is prepared by a conventional shot blast method or other suitable method. It is a process of removing the abrasive | polishing agent, dust, etc. which were on the surface of (1).

Heating process

The heating step is to mount the steel pipe (1) pre-treatment in the pre-treatment step to the upper portion of the trolley 20, to be transferred into the heating furnace 30 of the heating device 3, and then a plurality of inside the heating furnace (30) It is a process of heating the steel pipe 1, simultaneously operating the burner 32 and raising the temperature in the heating furnace 30. As shown in FIG. At this time, the heating temperature is a coating powder, which is the material to be coated, is heated on the surface of the steel pipe and heated to a temperature higher than the melting temperature of the coating powder, that is, a normal coating.

1st coating process

The said 1st coating process is a process performed by the powder coating by spraying in the state which rotates the steel pipe 1 heated by the above-mentioned heating process.

First, prior to performing the first coating process, the heated steel pipe 1 is lifted by a crane when passing through the heating furnace 30 of the heating device 3, and a pair of pairs installed on the upper portion of the trolley 20 are provided. The jig device 5 is fixed to the length of the steel pipe 1 at a suitable interval with the brake 50a fixed therein, and then the steel pipe 1 is moved to a position where the jig device 5 can be easily fixed. .

At this time, when the height adjustment of the fixing member 58, the jig 57 is installed together with the rotary drive 52 according to the diameter or shape of the steel pipe (1) can be adjusted by operating the hydraulic cylinder (50e) for height adjustment. .

Next, to fasten both ends of the steel pipe (1) to the jig device (5) to the buckle angle so as to match the angle of inclination of the ends of the steel pipe 1 (that is, the angle between the two ends of the steel pipe 1 according to the shape of the release pipe or straight pipe) Adjust the inclination angle between the support plate 56 and the rotary reciprocating member 53 by turning the adjuster 59 of the phosphorus angle adjusting member 55, and the fixing member 58 on the support plate 56 is provided on both sides of the steel pipe 1. In this state, the piston rod 53c of the hydraulic cylinder 53a is operated to move the fixed member 58 to the end thereof so as to coincide with the end thereof. 1) After fixing the both ends of the steel pipe (1) by operating a number of jigs (57) installed on the holding member 58 to adhere to the end and to hold the inner surface of both ends of the steel pipe (1) to separate the steel pipe (1) from the crane Let's do it.

After fixing the steel pipe 1 to the jig device 5 on the trolley 20 as described above, the trolley 20 is driven to enter the inside of the first coating apparatus 6 and rotated by the driving motor 52a. When the driving body 52 is rotated, the rotary reciprocating member 53 engaged with the rotary driving body 52 through a pair of spline portions 52b and 53b, and the support plate connected to the rotary reciprocating member 53 ( 56, the fixing member 58 coupled to the supporting plate 56, the jig 57 and the steel pipe 1 fixed by the jig 57 are also rotated.

As such, when the powder epoxy coating material powder is sprayed from a plurality of nozzles 62 installed on both sides of the coating chamber 60 while the steel pipe 1 is rotated, it uniformly contacts the surface of the heated steel pipe 1. Since the powder which contacts the surface of the steel pipe 1 adheres while melting by the heat which the steel pipe 1 has, the primary coating is performed. When the primary coating is completed in this way, the powder spraying through the nozzle 62 and the rotation of the steel pipe 1 are stopped.

2nd coating process

The second coating step is to drop the modified polyethylene powder having good adhesion from the top of the heated steel pipe (1) while maintaining the rotation of the powder-epoxy layer coated steel pipe (1) in the first coating process (1) The modified polyethylene powder, which is in contact with the surface of the epoxy layer on the outer powder, and the powder of the epoxy layer on the outer surface of the steel pipe (1) is attached while being melted by the heat of the steel pipe (1) to form a modified polyethylene adhesive layer. It is a process.

In detail, since the steel pipe 1 coated with the powder epoxy layer in the first coating process is mounted on the jig device 5 on the trolley 20, the trolley 20 is moved into the coating chamber 70. And stop it at a predetermined position, and then unwind or wind the wire 85c with the forward and reverse motor 85a to adjust the height of the powder applying mechanism 76 in the second coating apparatus 7, When the driving motor 52a is driven, the components are interlocked so that the steel pipe 1 is rotated as in the operation of the first coating process. As described above, the powder coating mechanism 76 of the powder coating mechanism 76 is rotated. Opening the opening and closing slot 83 and naturally dropping the modified polyethylene powder having good adhesion, the secondary coating is made because the powder in contact with the surface of the steel pipe (1) that was maintained in the heated state is melted and attached. When the secondary coating is completed as described above, the opening and closing slots 83 of the powder applying mechanism 76 are blocked to stop the dropping of the powder and the rotation of the secondary coated steel pipe 1.

At this time, the powder that is not attached to the surface of the steel pipe (1) as it is, falling into the trolley 20 of the lower portion of the steel pipe (1) and the powder collection space 78 located in the lower portion of the rail 10 is a two-way screw conveyor (79a) The powder coating mechanism 76 is gathered into the central compartment 79b by the following, and passes through the screw conveyor 86, the bucket conveyor 87, the screw conveyor 88, and the corrugated pipe 89, which are the feeding mechanism 80. To be supplied. Therefore, the modified polyethylene powder is circulated and recycled until it is deposited on the surface of the steel pipe 1.

3rd coating process

The third coating process is a modified polyethylene adhesive layer on the outside of the steel pipe (1) when the polyethylene resin powder is naturally dropped from the top of the steel pipe (1) while continuously rotating the steel pipe coated with the modified polyethylene adhesive layer in the second coating process The polyethylene resin powder, which is in contact with the surface and thus touches the surface of the modified polyethylene adhesive layer on the outside of the steel pipe 1, is attached while being melted by the heat of the steel pipe 1, thereby forming a polyethylene resin layer.

In detail, the steel pipe 1 coated with the modified polyethylene adhesive layer in the second coating process is mounted on the jig device 5 on the trolley 20 similarly to the first and second coating devices 6 and 7. Since the trolley 20 is moved to the inside of the coating chamber 70 and stopped at a predetermined position, the powder coating mechanism in the third coating apparatus 8 is unwound or wound by the forward and reverse motor 85a. By adjusting the height of 76 and driving the drive motor 52a of the jig device 5, the components are interlocked together to rotate the steel pipe 1 as in the operation of the first coating process. When the opening and closing slot 83 of the powder applying mechanism 76 is rotated while the (1) is rotated, and the modified polyethylene powder having good adhesion is naturally dropped, the powder which contacts the surface of the steel pipe (1) that has been kept heated is melted. It is attached so that the third coating is made. When the tertiary coating is completed as described above, the opening and closing slots 83 of the powder coating device 76 are blocked to stop the drop of powder and the rotation of the tertiary coated steel pipe 1.

At this time, the powder that is not attached to the surface of the steel pipe (1) as it is, falling into the trolley 20 of the lower portion of the steel pipe (1) and the powder collection space 78 located in the lower portion of the rail 10 is a two-way screw conveyor (79a) The powder coating mechanism 76 is gathered into the central compartment 79b by the following, and passes through the screw conveyor 86, the bucket conveyor 87, the screw conveyor 88, and the corrugated pipe 89, which are the feeding mechanism 80. To be supplied. Therefore, the polyethylene resin powder is circulated and recycled until it is deposited on the surface of the steel pipe 1.

Cooling process

The cooling process is to transfer the steel pipe 1, the coating is completed in the third coating process to the cooling device (9) to cool by a method such as immersing in the cooling water tank or spraying the cooling water in the steel pipe, the structure of the coated film This process is to complete the coating by primary to tertiary coating by forming a dense and uniform thickness.

In other words, the steel pipe 1 having completed the third coating in the above-described third coating process is separated from the jig apparatus 5 in the reverse order of the process of fixing the steel pipe 1 to the jig apparatus 5 in the first coating process, and then the crane Move to the cooling device (9) by using a forced cooling of the steel pipe (1) is completed tertiary coating.

As described above, the three-layer coating method and apparatus of the present invention can be applied to the case of the release pipe corresponding to the pipe fittings as well as the straight pipe form of the steel pipe, and can be universally applied to the shape of various steel pipes in small quantities. There is an advantage.

In addition, since the apparatus required for performing the coating operation is relatively simple and does not occupy a large installation space, it is possible to greatly reduce the installation cost of the facility compared to the conventional extrusion method, and consists of the first to third layers Even if the coating layers are formed sequentially, there is an advantage that each layer can be welded firmly and reliably.

In addition, it has excellent effects such as smooth and effective coating work by adopting a jig device that can be used universally regardless of straight pipe or deformed pipe while rotating while holding both ends of steel pipe during coating work. .

Claims (11)

A pretreatment step of removing abrasives and dust attached to the steel pipe, a heating step of heating the steel pipe passed through this pretreatment step, a coating step of forming a coating layer by applying a coating powder to the surface of the steel pipe subjected to this heating step, In the coating method of a steel pipe comprising a cooling step of cooling the steel pipe passed through the coating process, The first coating process of forming a powder epoxy layer on the surface of the steel pipe by powder coating by spraying the coating process while rotating the steel pipe, the steel pipe heated while rotating the steel pipe coated with the powder epoxy layer by the first coating process A second coating step of forming a modified polyethylene adhesive layer while the modified polyethylene powder dropped on the surface of the powder epoxy layer of the steel pipe by welding the modified modified polyethylene powder from the upper portion of the coating and the modified polyethylene adhesive layer is coated in the second coating process. Characterized in that the third coating step of sequentially forming the polyethylene resin layer while the polyethylene resin powder is dropped on the modified polyethylene adhesive layer by welding the polyethylene resin powder from the top of the steel pipe in the state of rotating the steel pipe 3 layer coating method of steel pipe by powder welding type. A pretreatment device for removing abrasives and dust attached to the steel pipe, a preheater for pretreating the steel pipe by the pretreatment device, and then heating the steel pipe; a coating device for forming a coating layer on the surface of the steel pipe heated by the heating device; In the coating device comprising a cooling device for cooling the steel pipe is coated by the coating device, The coating device is a pair of jig device that is rotated in a state where the steel pipe is mounted and seated on the upper portion of the trolley, a first coating device for forming a powder epoxy layer as a first coating layer, and a modified polyethylene as a second coating layer 3. A three-layer coating apparatus for steel pipes by powder welding, comprising a second coating apparatus for forming an adhesive layer and a third coating apparatus for forming a polyethylene resin layer as a third coating layer. The method of claim 2, The heating device has an opening and closing door is installed at the inlet and outlet of the heating furnace, a plurality of burners are installed on both sides of the heating furnace, a rail passing through the third coating device of the coating device at the inlet of the heating furnace is installed 3. The three-layer coating apparatus for steel pipes by powder welding, characterized in that a trolley having a drive mechanism is installed on this rail. The method of claim 2, The trolley is a three-layer coating device of the powder welded steel pipe, characterized in that a plurality of wheels rotatably installed on the frame so as to move on the rail and a drive motor is installed in some of these wheels to enable self-driving of the trolley. The method of claim 2, The jig device is provided with a fixed frame which is installed on the upper side of the base having the brake and the housing is formed on the upper side, a rotary drive body which is rotatable by a drive motor inside the housing of the fixed frame and has a spline portion formed therein, and the rotary drive A spline part formed on the outer surface to be engaged with the spline part of the sieve, and a rotating reciprocating member connected to the front and rearward by a hydraulic cylinder at the rear, and connected to the front end of the rotating reciprocating member through a hinge part, Powder welding type, characterized in that it consists of a support plate connected to the angle control by the angle adjustment member, and a fixed member provided with a plurality of jigs so that the slide movement is installed on the outside of the support plate and the radial expansion or contraction from the center is installed Three layer coating device of steel pipe by. The method of claim 5, The angle adjusting member is a three-layer coating device of the steel pipe by the powder weld type, characterized in that the turn-on buckle is fastened to the screw on both sides of the regulator. The method of claim 5, The fixed frame has a lower member fixed to an upper part of the base, and an upper member having a housing on the upper end of the lower member can be lifted and liftable between the upper member and the base such as a rotary drive member and a reciprocating reciprocating member. Three-layer coating device for steel pipes by powder welding, characterized in that the hydraulic cylinder for height adjustment is installed. The method of claim 2, The first coating device of the coating device is a three layer of powder welded steel pipe, characterized in that a plurality of nozzles for spraying the coating powder on both sides of the coating chamber is installed and a rail for moving the bogie is installed at the bottom. Coating equipment. The method of claim 2, The second and third coating apparatuses of the coating apparatus are provided with a fixing frame extending from the upper side of the coating chamber and having arms extending from the upper side of the coating chamber, and adjusting the height while hanging on the lower arm of the fixing frame. A powder applying mechanism installed to enable the powder coating mechanism, a collecting mechanism having an inclined surface at the lower portion of the coating chamber, and having a space for collecting powder, a feeding mechanism for feeding powder collected in the collecting mechanism to the powder coating apparatus, and a bottom of the coating chamber. Three-layer coating device for steel pipes by powder welding, characterized in that the rail is installed for movement of the bogie. The method of claim 9, The powder coating mechanism is installed with a winding rod which is rotated by a horizontal reversing motor, and connected to the wire end wound on the winding rod to enable height adjustment by winding and unwinding the wire according to the direction in which the winding rod is rotated. The two-way screw conveyor is installed in the inner space to spread the powder evenly in the longitudinal direction. The screw conveyor is driven by a drive motor, and the opening and closing slot of the powder welded steel pipe is installed at the lower part of the screw conveyor. 3 layer coating equipment. The method of claim 9, The collection mechanism is a bidirectional screw conveyor is installed so that the powder collected in the space for collecting powder is collected in the center, and a partition space for collecting and feeding the powder in the center of the powder collecting space is provided, the feeding mechanism is The screw conveyor rear end installed in the compartment is connected to the bucket conveyor installed in the rear of the column, and the screw conveyor is installed at the upper end of the bucket conveyor horizontally up to the upper portion of the powder coating mechanism. The three-layer coating device of the powder welded steel pipe, characterized in that the corrugated pipe is installed so that the end and the upper central portion of the powder coating mechanism is in communication with each other.